1. Field of the Invention
The present invention relates to a printer, facsimile apparatus, copier, multiplex machine or similar electrophotographic image forming apparatus. More particularly, the present invention is concerned with an image forming apparatus capable of collecting toner left on an image carrier after image transfer and returning it to a developing unit in the form of an air-toner mixture for reuse.
2. Discussion of the Background
It is a common practice with an electrophotographic image forming apparatus to form a latent image electrostatically on an image carrier, develop the latent image to produce a corresponding toner image, transfer the toner image to a paper or similar recording medium, and fix the toner image on the paper. There has been proposed an image forming apparatus including a toner recycling mechanism for collecting, after the transfer of the toner image from the image carrier to the paper, toner left on the image carrier with a cleaning unit, and conveying the collected toner to a developing unit or a collected toner storing section provided independently of the cleaning unit. The conventional toner recycling mechanism is implemented by any one of the following systems:
(1) a system communicating the toner outlet of the cleaning unit to the developing unit or the collected toner storing section by a piping in which a coil screw is accommodated, and causing the coil screw to convey the collected toner to the developing unit or the storing section, as taught in, e.g., Japanese Patent Laid-Open Publication No. 6-175488; PA1 (2) a system in which the collected toner storing section is positioned below the toner outlet of the cleaning unit, so that the collected toner can be transferred to the storing section mainly due to its own weight; and PA1 (3) a system using a powder screw pump, generally referred to as a Mono pump, having a rotor for driving the toner of a developer in its axial direction and a stator formed with a passage surrounding the rotor and contacting the rotor in the passage, and air feeding means for fluidizing the developer to be conveyed by the screw pump; the toner is conveyed via a flexible piping in the form of an air-toner mixture, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 7-219329.
The above system (1) has some problems left unsolved, as follows. The coil screw must be extended as far as the vicinity of the developing unit or the collected toner storing section. Because the conveyance path must be free from bends in order to guarantee the rotation of the coil screw, it is necessary to arrange only a linear path or a gently curved path. The toner storing section should preferably be positioned below the toner outlet of the cleaning unit. A frictional load acting between the coil screw and the piping while the screw is in rotation is heavy and increases a torque for rotating the screw, rendering long-distance conveyance difficult to achieve. In addition, the above load causes a stress to act in the toner being conveyed and thereby causes the toner to cohere or to melt due to heat. The system (1) is not practicable without resorting to bulky and sophisticated structural elements and is therefore undesirable from the standpoint of durability and easy maintenance. Moreover, because the conveying device is limited in position, the image forming apparatus is bulky, complicated and expensive.
The system (2) allows the toner conveying device to convey the collected toner relatively easily. However, because the collected toner storing section or the developing unit must be arranged substantially integrally with the cleaning unit, the system (2) is applicable only to low speed machines, i.e., user-oriented printers and copiers, due to limitations on position and the amount of toner which can be stored.
The system (3) capable of conveying stored toner or collected toner with the screw pump via, e.g., a flexible piping frees the toner storing section and toner collecting section from positional limitations, and therefore allows the toner to be surely conveyed with a simple construction. Because the air feeding means fluidizes the toner, the toner can be stably conveyed by the conveying device in the form of an air-toner mixture. The toner is therefore free from a stress and prevented from cohering or melting due to heat. However, the problem with the screw pump is that because the rotor formed of metal slides on the stator formed of rubber, the inside diameter of the stator increases little by little due to wear and creep. As a result, the amount of bite between the stator and rotor decreases and brings about the reverse flow of air to the inlet side of the screw pump via the gap between the stator and the rotor after the delivery from the pump. Such a reverse flow obstructs the conveyance of the toner. The screw pump is therefore determined to have reached the end of its life when the reverse flow occurs. It follows that the life of the screw pump is shorter than the lifes of the other parts, making the system (3) difficult to practice. While a greater amount of bite between the rotor and the stator may be set beforehand, such a scheme would bring about the blocking and deterioration of the collected toner due to temperature elevation ascribable to friction to act between the rotor and the stator during continuous operation. The blocking and deterioration of the toner would lower image quality and toner conveyance quality. Moreover, while the screw pump is continuously operated, air fed under pressure from the air pump continuously enters the developing unit and causes the toner to fly about by increasing the internal pressure of the developing unit.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 58-163978, 61-67074, 5-27649, 6-342240, 7-77906, and 7-104631.